God’s Messenger received the revelation, “Your wives are a tilth to you, so come to your tilth . . .” (Al-Qur’an 2:223) that means from in front or behind, but avoid the anus and intercourse during menstruation.
Abu Sa'id al-Khudri reported God's Messenger as saying, “The most serious breach of trust in God’s sight on the day of resurrection ...” A version has, “Among those who will have the worst position in God’s sight on the day of resurrection is the man who has intercourse with his wife, and she with him, and then spreads her secret.”*
* i.e. talks about the subject to others, or tells people about defects or beauties he has found in her.
